1. Field of the Technology
The present application relates generally to three terminal magnetic sensors (TTMs) suitable for use in magnetic heads, and more specifically to TTMs having base lead layers in-plane with collector substrate materials.
2. Description of the Related Art
Magnetoresistive (MR) sensors have typically been used as read sensors in hard disk drives. An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used as a MR read head for reading data in magnetic recording disk drives, operates on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which causes a change in resistance of the read element and a resulting change in the sensed current or voltage.
A three terminal magnetic (TTM) sensing device of a magnetic head may comprise a spin valve transistor (SVT), for example, which is a vertical spin injection device having electrons injected over a barrier layer into a free layer. The electrons undergo spin-dependent scattering, and those that are only weakly scattered retain sufficient energy to traverse a second barrier. The current over the second barrier is referred to as the magneto-current. Conventional SVTs are constructed using a traditional three-terminal framework having an “emitter-base-collector” structure of a bipolar transistor. SVTs further include a spin valve (SV) on a metallic base region, whereby the collector current is controlled by the magnetic state of the base region using spin-dependent scattering. Although the TTM may involve an SVT where both barrier layers are Schottky barriers, the TTM may alternatively incorporate a magnetic tunnel transistor (MTT) where one of the barrier layers is a Schottky barrier and the other barrier layer is a tunnel barrier, or a double junction structure where both barrier layers are tunnel barriers.
FIG. 1 illustrates TTM operation associated with a conventional SVT 100 which has a semiconductor emitter region 102, a semiconductor collector region 104, and a base region. 106 which contains a spin valve. The semiconductors and magnetic materials used in SVT 100 may include an n-type silicon (Si) material for emitter 102 and collector 104, and a Ni80Fe20/Au/Co spin valve for the region 106. Energy barriers, also referred to as Schottky barriers, are formed at the junctions between the metal base 106 and the semiconductors. It is desirable to obtain a high quality energy barrier at these junctions with good rectifying behavior. Therefore, thin layers of materials (e.g. platinum and gold) are oftentimes used at the emitter 102 and collector 104, respectively. Moreover, these thin layers separate the magnetic layers from the semiconductor materials.
A TTM operates when current is introduced between emitter region 102 and base region 106, denoted as IE in FIG. 1. This occurs when electrons are injected over the energy barrier and into base region 106 by biasing the emitter such that the electrons are traveling perpendicular to the layers of the spin valve. Because the electrons are injected over the energy barrier, they enter base region 106 as non-equilibrium hot electrons, whereby the hot-electron energy is typically in the range of 0.5 and 1.0 eV depending upon the selection of the metal/semiconductor combination. The energy and momentum distribution of the hot electrons change as the electrons move through base region 106 and are subjected to inelastic and elastic scattering. As such, electrons are prevented from entering collector region 104 if their energy is insufficient to overcome the energy barrier at the collector side. Moreover, the hot-electron momentum must match with the available states in the collector semiconductor to allow for the electrons to enter collector region 104. The collector current IC, which indicates the fraction of electrons collected in collector region 104, is dependent upon the scattering in base region 106 which is spin-dependent when base region 106 contains magnetic materials. Furthermore, an external applied magnetic field controls the total scattering rate which may, for example, change the relative magnetic alignment of the two ferromagnetic layers of the spin valve. The magnetocurrent (MC), which is the magnetic response of the TTM, can be represented by the change in collector current normalized to the minimum value as provided by the following formula: MC=[IPC−IAPC]/IAPC, where P and AP indicate the parallel and antiparallel state of the spin valve, respectively. Since these types of devices have small output currents due to the small differences between the two Schottky barrier heights of the semiconductor, MTT and double Tunnel embodiments are generally preferred.
In FIG. 2, a cross-sectional view of a conventional TTM 200 of the MTT type is shown. TTM 200 of FIG. 2 has a base region 215, a semiconductor collector substrate 220 which is adjacent base region 215, an emitter region 205, and a barrier region 210 which separates emitter region 205 from base region 215. Base region 215, barrier region 210, and emitter region 205 form a sensor stack structure 201 of TTM 200. A first Schottky barrier 211 is formed at the interface between base region 215 and semiconductor collector substrate 220. Also, a second Tunnel barrier 212 is formed within sensor stack structure 201 at the interface between emitter region 205 and base region 215 at barrier layer 210 in a single deposition step. An emitter conductive via 235 is formed adjacent emitter region 205 of sensor stack structure 201, a collector conductive via 236 is formed adjacent semiconductor collector substrate 220, and a base conductive via 234 is formed by etching the sensor stack layer structure down to base region 215. Insulator materials 250 surround the various structures of TTM 200.
In FIG. 3, a cross-sectional view of an alternative conventional TTM 300 of the SVT type is shown. TTM 300 of FIG. 3 has a base region 315, a semiconductor collector substrate 320 which is adjacent base region 315, an emitter region 305, and a barrier region 310 which separates emitter region 305 from base region 315. Base region 315, barrier region 310, and emitter region 305 form a sensor stack structure 301 of TTM 300. A first Schottky barrier 311 is formed at the interface between base region 315 and semiconductor collector substrate 320 to define the geometry of base region 315. Also, a second Tunnel barrier 312 is formed at least partly over base region 315 at barrier layer 310 to therefore form emitter region 305 with an ex-situ process. An emitter conductive via 335 is formed adjacent emitter region 305 of sensor stack structure 301, and a collector conductive via 336 is formed adjacent semiconductor collector substrate 320. A base conductive via 334 is formed by etching the sensor stack structure down to base region 315. Insulator materials 350 surround the various structures of TTM 300.
In general, sensor stack structures are fragile and susceptible to damage due to ion bombardment and chemical exposure during manufacturing steps such as those used in the formation of conductive vias for connecting TTM base regions to their terminals. Metal layers involved in TTMs are generally within 5 nm and 10 nm thick, such that subtractive processes usually required to shape these devices can change the magnetic properties of the metal layers. Furthermore, in conventional TTMs 200 and 300 of FIGS. 2 and 3, base regions 215 and 315 are formed relatively longer than their respective emitter regions 205 and 305. This difference in length is necessary to facilitate the formation of base region conductive vias 234 and 334 while avoiding damage to sensor stack structures 201 and 301 associated with ion bombardment and chemical exposure. As a result, the trackwidths are unnecessarily large due to the relatively long length of the base regions. It is advantageous to form very thin and narrow base regions in TTMs for increased areal recording densities and smaller trackwidths.
Accordingly, there is a need to solve these problems so that TTMs may be suitable for use in magnetic heads and other devices.